Sound recording amplifier circuits



SOUND RECORDING AMPLIFIER CIRCUITS Filed Dec. 1,- 19:59

7; RECTIFIER AMPL/ #152 7b Iva/s Esoucr/o/v Slit/r7525 7'0 6A1. win/0m k E E g 8 S Q Z 8 y a Q 8 g a .Y Q q Q Q Q 0 & u m 3 Q 4 2 I M 1 INPUT SIGNAL f/vPur SIGNAL INPUT SIGNAL gi til W Patented Sept. 8, 1942 SOUND RECORDING AMPLIFIER CIRCUIT Barton Kreuzer, Los Angeles, Calif assignor to Radio Corporation of America, a corporation of Delaware Application December 1, 1939, Serial No. 307,129

1 Claim.

This invention relates to sound recording cirsuits and particularly to automatic gain control amplifiers therefor wherein different rates of gain or amplification are obtained over selected portions of the signal amplitude range being recorded.

In the art of sound recording, it has been founddesirable to record different portions of a signal range at different ratios of amplification. One reason why this is desirable is because of the limited range of the film track area compared with The principal object of the invention, therefore, is'to improve the recording characteristics of a recordingsystem.

Another object of the invention is to automatically control the transmission ratio of signals being recorded.

A further object of the invention is to automatically vary the gain of an amplifier at different signal input levels. p

A further object of the invention is to provide a plurality of selective control circuits for controlthe range of certain signals being recorded, such as those from a large orchestra. In this instance, the upper amplitude range is reduced by introducing compression, while the lowerlevels may be expanded for noise-reduction elimination. However, it is not desirable to expand the extremely low levels, since their expansion would increase the acoustic or set noises which are always present. Furthermore, for certain types of recording wherein dialogue composes the major portion of the recording, the dialogue may be recorded linearly; that is, directly proportional to the variations in change of the signal, while higher level sounds may be compressed. However, where certain dramatic efiects are introduced, producing high momentary peaks, it is necessary that these peaks .be reduced to prevent listener annoyance in theatres where thereproduced level is to db. higher than normal. The invention also lends itself to the automatic introduction of certain types of dramatic effects. I

Ordinarily, a mixer, by manual control and after a rehearsal to acquaint himself with the type and trend or the'sound being recorded, controls the amplifier to not only bring the signal within the film range, but also to produce expansion over certain amplitude ranges, a linear relationship between signal and film impression over other ranges, and a. compression effect over the higher ranges. The present invention, however, contemplates accomplishing these results automatically for a definite type Olf signal. For instance, one automatic characteristic may be such that at extremely small levels, normal amplification occurs, then, above a certain threshold value, expansion may occur up to another amplitude level, then linear recording is again introduced up to another predetermined level above which compression at a certain ratio may occur. This automatic control is eil'ected-by a plurality of parallelconnected rectifiers whose outputs are connected in series tor controlling the bias of an amplifier between the sounce of signals and the recording eq p nt.

ling the gain of an amplifier, the control circuits having their inputs and outputs connected to the Although the novel features which are believed to be characteristic of this invention are pointed out with particularity in the claims appended herewith, the manner Olf its organization and the mode of its operation will be better understood'by referring tothe following description read in conjunction with the accompanying drawing forming a part thereof in which Figure 1 is a diagrammatic arrangement of the amplifier portion of a sound recording system;

and l 7 Figures 2, 3 and 4- are graphs showing different transmission or amplification characteristics obtainable with the circuit of Fig. 1.

Referring now to Fig. 1, signals may be detected by different types of piclcup devices, one of which may be a microphone such as shown at 5. The detected signals are amplified by an amplifier 6 "and fed to a controlled amplifier I through a transformer 8. The amplifier 'l is composed of a pair or push-pull variable mu tubes 10 and II having grid circuits including the secondary of 7 I transformer 8 and loading resistances I2. A biasing resistance I3 and bypass condenser 14 are shown in the grid, cathode circuit. It is to be understood that, although a push-pull amplifier is shown, a single tube amplifier may also be employed. The output of. tubes l0 and II is divided and fed to a sound-recording galvanometer (not shown) over conductors l6 and to a rectifier amplifier II, the output of which is fed to noisereduction shutters (not shown) over conductors IS. The usual sources of potential for the anodes and screen grids and heating source forthe cathodes are employed but have not been shown for the purposes of explanation! This general type ofsound recording system is disclosed in detail in 'Patents2,l02,7'l6, 2,102,777, and. 2,102,778.

The output of tubes 10 and l I is further divided and fed into three parallel-connected rectifier circuits A, B and C, throughrespective trans dormers 23, 2| and 22 fed from conductors 23. Circuit A contains a rectifier 28, a biasing source of potential 21 and a resistor 28 shunted by a ypass condenser 23. Rectifier circuit B contains a rectifier 3i a biasing source of potential 32, and a resistance 33 shunted by a bypass condenser 33, while rectifier circuit C is composed of a rectifier 33, a biasing source of potential 31, and a resistance 33 shunted by a bypass. condenser 39. It will be noted that the biasing circuit for the tubes [3 and II is from the grids of tubes Wand ll, through resistances i2, over conductor 32, through resistance 23, over conductor 33, resistance 33, conductor 33, resistance 38 and through ground to resistance l3 and the cathodes or the tubes I8 and II. In the rectifier circuits A, B and C, the

polarities d. the various circuit elements have,

a certain amplitude or threshold level from tubes I8 and ii must be reached. When this threshold value is reached, current will flow through the resistance 28, creating a voltage in a direction indicated by the polarity marks. This voltage variation reduces the normal negative bias on the tubes I 3 and H, and increases their gain or amso that the voltage produced across it is in opposition to the voltage across resistance 28, and will thus either neutralize or at least reduce the biasing effect of the resistance 28' according to the value of these resistances.

As the signal continues to increase, the'threshold value of rectifier unit C is reached, and resistance 38 becomeseifective to either neutralize or to actually negatively increase the bias on tubes i0 and II. Thus, it is obvious that by adjustment of the biasing sources 21, 32 and 31, and resistances 28, 33 and 33, various degrees of expansion and compression or linear amplification may be obtained from the tubes In and II, and the signal being impressed upon the film will vary accordingly.

To further illustrate the characteristics obtainable with the invention, reference is made to Figs. 2, 3 and 4, in which the relationship between the-input signal and the reproduced output from the film is shown, the 45 reference curve representing linear transmission or amplification In Fig. 2, normal amplification occurs from zero to point :r," and thus certain "set noises and other inherent acoustic noises are maintained at a low level and do not become annoying. The

also expanded, they are masked by the signal at the threshold value for rectifier unit C, because potential sources 32 and 31 have been made equal. At the level of point 1/. therefore, current flows through both resistances 33 and 33, and their voltage variations are additive to overcome the normal reduction in bias introduced by resistance 28 and to make the bias go negative. Thus, for amplitude levels above point "1)," compression is introduced in the signal, as shown by the remainder of the curve, and the 1 211 peaks are thus brought within the film range as well sis obtaining the other advantages of compress on.

Referring now to Fig. 3, the threshold value for rectifier unit A is made the same as in Fig. 2. so for very low signals such as set noises, background noises, etc., normal amplification is provided. Similar to Fig. 2, expansion begins at point 4:, since the current flowing in resistance 28 will reduce the bias on tubes I3 and II as in Fig. 2. At the point 11" level, however, bias 32 onrectifier B has been adjusted to, send sumcient current through resistance 33to just neutralize the effect of, resistance 28 so that, between the level at point and the level at a point z, the amplifier lo- -ll is again linear. By adjustment of bias 31, point "z is the threshold amplitude limit for rectifierunit C and, when it becomes efiective, it increases the negative bias and introduces compression, as shown-by the re-' mainder of the curve.,-

Referring to Fig. 4, the threshold value of-rectifier A is shown at point '21:, and the threshold value of rectifier unit B is shown at point "1!," similar to Figs. '2 and 3. However, in this case.

the value of resistance 33 is such as not to fully neutralize the effect of resistance 23, so that there still remains some expansion of the signal betweenthe point 1 and the point z." At point 2, however, the rectifier unit C becomes effective, fully overcoming the eiiect of resistance 28 and introducing compression in the circuit, as shown by the remainder of the curve.' Itis to be understood that the points 2:1? "11 and "z" may be positioned at any predetermined amplitude depending on the values of the rectifier biasing potentials, while the ratio of amplification ma be-controlled by the values of the biasing tial ones have been made according to the type the level of point z. This condition exists for increases in amplitude up to point y," which is the threshold value for rectifier unit B and also of signal being recorded. The inputs to rectifiers 26, 3| and 33 mayalso be provided with voltage controls to'shift the breakaway points and the rate of gain of the amplifier 1, it being understood that the input circuits may have a series relationship. The start and release times of the expansion and compression occurring in the system are functions of the internal resistances of rectifiers 26, 3| and 38, the values of resistances 28, 33 and 38 and the values of bypass condensers 28, 33 and 39.

What I claim as my invention is:

A'sound recording amplifier circuit comprising a push-pull variable gain amplifier, a plurality of parallel-arranged rectifiers having their inputs connected in parallel, means for separately biasing each of said rectifiers to provide different operating points, one of said rectifiers operating over a certain range of signal levels, two of said .rectiflers operating over another range of signal levels and all of said rectifiers operating over a third range of signal levels, means for connecting the output circuits of said rectifiers in series, said means including a resistance in each of the output circuits of said rectiflers, said resistances having difierent values, means individual to each resistance for controlling the timing characteristic of each of said rectifier output circuits, and

10 said different biases.

BARTON KREU ER. 

